The truth of running of the clamped workpiece is especially important with respect to the machining accuracy which the machine tool in question can achieve. Factors which can cause an undesirable decrease in the truth of running include, first, the presence of an angle between the workpiece axis and the spindle axis and, second, the presence of an offset between the workpiece axis and the spindle axis. This offset is also referred to as the “eccentricity” of the clamped workpiece. The present description pertains exclusively to the eccentricity of the clamped workpiece.
The known workpiece headstock (WO 2004/007128 A1), from which the present invention proceeds, is equipped with a spindle and a clamping device, which is connected to the spindle and which is designed as a spring collet chuck. The spring collet chuck is connected to the spindle by way of a flange joint with straining screws. So that any eccentricity of the workpiece which may be present can be corrected during the alignment procedure, adjusting screws are provided circumferentially around the spring collet chuck, each of which bears radially against the spindle. To align the chuck, the straining screws of the flange joint are slightly loosened, so that, by appropriate actuation of the adjusting screws, the spring chuck can be adjusted radially with respect to the spindle. The flange joint therefore represents simultaneously an alignment interface, which makes it possible to align the clamping device. In addition, a comparable arrangement is provided between the two sections of the spring collet chuck. This arrangement is used to make any precision adjustments which may be needed to correct for the possible eccentricity of the workpiece to be clamped subsequently in the chuck.
The disadvantage of the known workpiece headstock is that the work of aligning the clamping device to eliminate the eccentricity of the clamped workpiece takes a great deal of time and requires a great deal of skill on the part of the operator. The alignments achieved with this type of adjusting device based on radial adjusting screws, furthermore, are not reproducible.
Another disadvantage of the known workpiece headstock is that the adjusting device is integrated into the clamping device. This makes it more difficult to manufacture the clamping device and also leads to design limitations in the realization of the adjusting device because of the limited space available. Insofar as the workpiece headstock is designed to accommodate several different clamping devices, each replaceable clamping device must be equipped with its own appropriate adjusting device. In addition, imbalances which can unfavorably affect the machining of the workpiece are almost always associated with this type of adjusting device.
A similar concept for aligning the clamping device for a tool is described in EP 0 882 535 A1. Here two sections of the clamping device are again connected to each other by way of a flange joint with straining screws. The clamping device is aligned to eliminate the eccentricity of the tool by slightly loosening the straining screws and by placing a special adjusting tool on the clamping device. By means of the adjusting tool, the two sections of the clamping device can be shifted toward each other in the radial direction with respect to the axis of the spindle. As in the above-described workpiece headstock, the success of the alignment procedure depends to a great extent on the skill of the operator.
It is also known that the eccentricity of a workpiece on a lathe can be adjusted effectively so that workpieces which include eccentric sections can be machined. The eccentricity of the workpiece can be adjusted here by rotating the two sections of the clamping device, which are connected by an alignment interface, with respect to each other around the spindle axis. This is done by using the spindle drive to turn the spindle, which thus turns one of the two sections of the clamping device. Simultaneously, a stationary mandrel holds the other section of the clamping device so that it cannot turn. The arrangement is set up so that the clamping device is loosened first, and then the alignment interface is loosened, so that the two sections of the clamping device can be rotated with respect to each other. The mandrel also has the function of ensuring that enough retaining force is being exerted on the workpiece and for this purpose exerts an axial force on the clamping device during the adjustment of the eccentricity.
The disadvantage of the workpiece headstock just described is again the fact that the adjusting device for aligning the clamping device is integrated into the clamping device. In addition, the truth of running which can be achieved with this adjusting device is limited.
Another known workpiece headstock of a machine tool (DE 89 15 435 U1) is equipped with a clamping device with a workpiece holder, which can be shifted freely in a direction perpendicular to the spindle axis. To guide the workpiece, two axially aligned steadies are arranged in front of the workpiece holder. The disadvantage here is that a comparatively large amount of effort is required to manufacture the clamping device. An additional support device is also required to prevent the workpiece holder from dropping under its own weight when the workpiece is being changed.
A fundamentally similar concept can be found in EP 1 419 852 A1, which describes a workpiece headstock with hydrostatic spindle bearings. The forward spindle bearing facing the clamping device is adjusted in such a way that it has a larger amount of radial bearing play than the rear spindle bearing. Here, too, the workpiece is guided by a steady, which is installed in front of the workpiece holder.
Finally, it should also be pointed out that any eccentricity of the clamped workpiece which may be present can also be corrected by automatic control means (DE 198 82 642 T1). Especially when workpieces of small dimensions are being machined, however, this variant imposes severe requirements on the drive engineering of the machine tool.